1. Field of the Invention
The present invention relates to a mechanical ultrasonic scanner for mechanically swinging a transducer element, thereby scanning the interior of a living body by a ultrasonic beam emitted from the transducer element, so that an image of the structure and movement of internal organs of the living body is displayed in real time.
2. Description of the Related Art
In a mechanical ultrasonic scanner, a transducer element is swingably supported in a housing. This transducer element radiates an ultrasonic beam while being swung by, e.g., a motor. Therefore, the inside of a living body is scanned by the ultrasonic beam. After scanning, the ultrasonic beam returned from the living body is detected by the transducer element. The detected ultrasonic beam reconstructs an image to obtain a tomogram.
The housing contains a liquid sound transmitting medium (e.g., a mineral oil). The transducer element is dipped in the sound transmitting medium. This sound transmitting medium has a property of easily transmitting an ultrasonic beam in a frequency range incident on a living body. Therefore, the ultrasonic beam radiated from the transducer element can be transmitted without being obstructed in the housing, and can be incident on the living body.
In order to reconstruct an image by the detected ultrasonic beam, a direction in which the ultrasonic beam is radiated and returned from/to the transducer element must be detected. Therefore, a swinging angle of the transducer element is conventionally detected by an optical encoder to obtain a radiating/returning direction of the ultrasonic beam.
In a liquid sound transmitting medium, however, light emitted from the optical encoder may be irregularly reflected. In addition, swinging of the transducer element causes the sound transmitting medium to flow, and irregular reflection of the light is enhanced. Furthermore, straight propagation of the light may often be interrupted by dust which floats in the sound transmitting medium. For these reasons, the light is not accurately detected, and the swinging angle of the transducer element is not often detected accurately. Therefore, a radiating/returning direction of the ultrasonic beam cannot be accurately obtained, and a reconstructed image may often be inaccurate.